With the advent of pulse oximetry, oxygen saturation measurements have
been simplifiehttp://www.altitudeclinic.com/images/graphcc1.gif
http://www.altitudeclinic.com/images/graphcc1.gifd. However, at high altitude, large fluctuations from breath
to breath have been observed.
Hence, in order to evaluate the performance of pulse oximetry, 20 normal
native residents of the bowl-shaped city of La Paz (3100-4100 m), with
mean age 20.03 ± 3.07, were studied in our laboratory at 3510 m.
Finger oximetry was performed on sitting subjects during 15 minutes, of
which the last 5 minutes were used for analysis. A computer hook-up via
serial port from a BCI portable oximeter plotted and stored the oxygen
saturation and pulse every 5 seconds. Subjects were prevented from observing
the results, in order to avoid them changing their breathing pattern. In
6 of these subjects, measurements were continuous while the door locks
of the Hyperoxic\Hypoxic Adaptation Chamber (HHAC) were closed and the
partial inspired oxygen tension (PIO2) was raised from breathing ambient
air (AA) (PIO2 = 94 mmHg) to simulated sea level (SSL) values (PIO2 = 150
mmHg) without changing the barometric pressure (PB = 494 mmHg) at 23 ºC.
After breathing 15 minutes in the chamber, oximetry recordings during the
last 5 minutes were used for analysis.
Maximum (max) and minimum (min) values, mean, standard deviation and
frequency distribution of oxygen saturation (SAT) as well as pulse of all
the subjects were calculated and plotted. Although individual variations
are evident, using finger oximetry, the average results show that significant
variations in oxygen saturation and pulse of native residents of high altitude
are diminished when exposed to simulated sea level in the HHAC.

This abstract is published in ACTA ANDINA V:2 (Special
issue I) 1966 and was presented during the 2nd World Congress on High Altitude
Medicine and Physiology held in Cusco, Peru.(Sept. 24-27, 1996).

INTRODUCTION

Pulse oximeters have been described as "remarkable among monitors in
that it involves no calibration, negligible time lag and infrequent false
negative data and requires no routine maintenance" (1). However, those
that have been at high altitude and used an oximeter, have found that the
readings had large variations, to the point that malfunction of the apparatus
was even considered. Nocturnal desaturation at high altitude has been studied
(2, 3, 4), but daytime oscillations have not been reported. Also,
during flight, large saturation variations between individuals have been
observed (5).

In order to study the saturation variations in natives living at 3510
m, measurements breathing ambient air and simulated sea level in the hyperoxic/hypoxic
adaptation chamber (HHAC) were recorded. This allowed evaluation
of the same pulse oximeter without long time spans as well as wheter the
same oximeter used under sea level partial inspired oxygen tension (PIO2)
would perform as expected.

METHODS

A BCI portable oximeter was hooked up via serial port to a computer
which plotted saturation and pulse every 5 seconds. A finger probe was
placed in the right index finger. The subjects were sitting inside the
hyperoxic\hypoxic adaptation chamber. The doors of the door lock were open
and each subject was breathing ambient air at 3510 m, with barometric pressure
of 495 mmHg and partial inspired oxygen tension (PIO2) = 94 mmHg. Following
15 minutes of continuous recording, the doors of the lock were closed and
the oxygen percentage in the chamber was raised to 32% that corresponds
to a sea level PIO2 of 150 mmHg at the same barometric pressure. Air recirculated
through a soda lime absorber, prevented CO2 rise. The temperature was kept
constant at 23 º Centigrade.
Twenty natives of high altitude (Age mean = 20.03 ± 3.07)
were studied breathing ambient air and 6 of those also under simulated
sea level conditions. Average and standard deviations as well as coefficient
of variance were calculated in both groups using a Qpro spreadsheet.

RESULTS:

The results are shown in table 1. Saturation averages of each group
varied from 95.4 % (maximum) to 89.0 % (minimum) during ambient air
conditions and changed to 98.4 % (maximum) to 97.0 % (minimum) during simulated
sea level conditions. Similarly, pulse variations can also be observed.
A frequency distribution (C) of the average of the last 5 minutes of observations
in the 6 patients breathing ambient air (A) and during hyperoxia in the
HHAC (B) is shown in fig. 1.

The troublesome reading of saturation in a pulse oximeter at high altitude
is evident and usually not an apparatus malfunction. Several reports on
the response of oximeters under low perfusion thresholds, profound hypoxia,
anemia, show manufacturer variations of response in pulse oximeters (6
,7, 8). However the variation at high altitude did not seem to be reported.
The evident decrease of the coefficient of variability when the subjects
were in the HHAC, shows that at sea level (acute exposure) the oximeter
performs as expected and therefor malfunction can be ruled out.

As expected, the reference values of pulse oximetry at high altitude,
are conflicting because not only are age, sex, race, different physiological
states such as sleep and health status involved, but also the different
altitudes. At 2640 m the mean saturation in children is reported at 91.1%
during sleep and 93.3 % in daytime activity (9).

Some variations of pulse oximetry in newborns at 1610 m is reported
and questioned for ‘what is normal?” (10). Although that study is only
at 1620 m, the variations in measurement of saturation may have given rise
to the question.

The reports that measurement of saturation can aid in the diagnosis
of pulmonary illness in children at high altitude (11, 12, 13) are interesting
but the variations we report should be taken into consideration. Quick
measurements of saturation can be deceiving. In our long experience with
high altitude, the resting steady state is difficult to achieve and a minimal
psychological unrest will affect it considerably. Changes in the respiratory
frequency and tidal volume, as in deep inspiration (and even talking) where
a 99% saturation at 3600 m can be reached, make it difficult to maintain
a steady state ventilation.

It is also important to consider that at 3510 m, the average saturation
being 92 %, one deep breath can increase the saturation to 99 % in some
subjects. The shape of the oxygen dissociation curve should also be taken
into account. Saturation fluctuations in the steeper portion are obviously
greater, however, ventilatory irregularities which are typical of high
altitude are the main cause. Furthermore, apneas may not only be present
during sleep, but also while being awake. This implies constant change
in blood pH, and CO2 elimination, with frequent shifts in the p50 of the
oxygen dissociation curve. Some authors report that at high altitude the
in vivo p50 is shifted to the left (14, 15). Others say that it is shifted
to the right (16, 17) and some that it remains the same (18). Still others
claim that measurements below 14,000 ft are left shifted and those above
14,000 feet right shifted (19). Saturation variations may be a reason why
reports of in vivo p50 have been conflicting.

When blood gases are about to be drawn, a hyper-ventilation or a hypoventilation
at this altitude, can cause great variations. In patients with chronic
mountain sickness that are suffering the triple hypoxia syndrome (20),
these fluctuations are in the steepest part of the curve.

Our bottom line advice is to be careful while measuring oxyhemoglobin
saturation with a pulse oximeter at high altitude.

Double click on images to see a larger graph

AB

C

Fig.1 Frequency distribution of the last 5 minutes of six high
altitude natives at 3510 m (Pb = 495 mmHg) breathing ambient
air PIO2 = 95 mmHg (A) and during hyperoxia PIO2 = 150 mmHg in the HAAC
(B). The average of both groups {n=20 for ambient air in blue and n=6 for
hyperoxia in red} are plotted in graph (C).